Stationary magnetically stabilized fluidized bed for protein separation and purification

ABSTRACT

Magnetically stabilized fluidized bed technology is utilized in conjunction with ion-exchange adsorption/desorption processes in a method and system for isolating proteins from cell lysate. The invention also includes a magnetizable, porous, ion-exchange particle, and a method for producing the same, for use with the stationary magnetically stabilized fluidized bed protein isolation process.

FIELD OF THE INVENTION

This invention relates to a method and system for performingchromatographic or adsorption/desorption isolations of proteinsutilizing magnetically stabilized fluidized beds. Ion-exchange particlesthat have been made magnetizable are held in a stationary bed subjectedto a radially uniform magnetic field as a solution containing proteinsis passed upwardly through such bed. Following adsorption of theproteins onto the ion-exchange sites of the particles, an ionic ordifferent pH solution is utilized to free the proteins from theparticles.

BACKGROUND OF THE INVENTION

A fluidized bed is created when a gas or liquid is passed upwardlythrough a bed of solid particles with sufficient velocity wherein thedrag forces of the gas or liquid counterbalance the gravitational forceson the particle and cause the bed to expand. A fluidized bed consists ofparticles that are completely submerged and levitated in the fluidizingfluid. In contrast, in a "packed" bed, the particles are fixed in spaceand have no translational freedom, due to their permanent contact withparticles (or walls) surrounding them.

Ronald E. Rosensweig was the first to investigate the possibility offorming a "stabilized" fluidized bed by utilizing magnetizable particlesand placing the system in a magnetic field. See R. Rosensweig,Fluidization: Hydrodynamic Stabilization With a Magnetic Field, 204Science, pp. 57-60 (1979). Rosensweig's research concentrated ongas/solid systems. In gas/solid fluidized beds--used in heterogeneouschemical reactions such as hydrocarbon cracking--bubble formationgreatly reduces the effectiveness of the process. Rosensweig discoveredthat by utilizing magnetizable particles in a radially uniform magneticfield, it was possible to create a "stabilized" fluid bed. Rosensweig,and others, have reported that the stabilization effect is more easilyaccomplished when the magnetic field runs parallel to the path of fluidflow.

A discussion of magnetically stabilized fluidized beds ("MSFB") inliquid/solid systems is found in J. H. Siegell, Liquid-FluidizedMagnetically Stabilized Beds, 52 Powder Technology, pp. 139-48 (1987).The effects of the magnetic stabilization are not as dramatic as thoseseen in the gas/solid systems, but nonetheless are quite significant.Siegell characterized four regimes in upwardly flowing solid particlebeds: packed, stable, random motion and boiling.

In the absence of a radially-uniform magnetic field, a system utilizingthe upward flow of fluid through a particle bed goes through threeregimes as the velocity of fluid flow is increased. The "packed" regimeis the same as in the presence of the magnetic field. At the point of"incipient fluidization"--where the velocity of the fluid creates dragforces that exactly counterbalance the gravitational effects on theparticles--the random motion regime begins. With increased fluid flowvelocity, the boiling regime can be seen.

The point of incipient fluidization is also the transition between thepacked and stable regimes in MSFB. This point is not affected by thestrength of the magnetic field applied. One can reach the stable regimeeither by first applying the magnetic field and then increasing the flowabove the point of incipient fluidization, or by applying a magneticfield to the bed in the random motion regime already above the point ofincipient fluidization.

When in the stable regime, the pressure drop in the bed remains constantwith increased flow rate, the void volume of the bed increases, yetthere is restricted motion of the particles due to the existence of themagnetic field. In the stable regime the bed of particles is clearlyfluidized (expanded and flowable), yet it lacks the random motiontraditionally associated with fluidized beds.

The effect of the magnetic field can be viewed roughly as creating amagnetic dipole in each particle, which causes it to become "sticky" ina direction parallel to the magnetic field lines. This produces whatamounts to the formation of chains of beads parallel to the axis of thebed. For a detailed mathematical/theoretical investigation of themechanism for MSFB, see Rosensweig et al., Continuum Modes of DiscreteSystems 4, O. Brulin and R. K. T. Hsieh, eds., North Holland Publishers,Amsterdam, 137-143 (1981).

References that describe the use of MSFB in conjunction with eitheradsorption/desorption or chromatographic separations are limited. Ofcourse, the use of magnetizable particles in biochemical systems isrelatively common. The references that disclose the use of MSFB havebeen restricted to affinity interactions.

Work described by Burns and Graves is directed towards a system usingcounter-current liquid/solid phase continuous affinity chromatography.See M. Burns and D. Graves, Continuous Affinity Chromatography Using aMagnetically Stabilized Fluidized Bed, 1 Biotechnol. Prog., pp. 95-103(1985), M. Burns and D. Graves, Application of Magnetically StabilizedFluidized Beds to Bioseparations, 6 Reactive Polymers, pp. 45-50 (1987);and M. Burns and D. Graves, Structural Studies of a Liquid-FluidizedMagnetically Stabilized Bed, 67 Chem. Eng. Comm., pp. 315-330 (1988).Rather than utilizing a stationary column of magnetizable particles,Burns and Graves anticipate using a system where the particles flowdownwardly as the solution flows upwardly.

A paper by Lochmuller and Wigman also deals with the use of MSFB andaffinity interactions. C. H. Lochmuller and L. S. Wigman, AffinitySeparations in Magnetically Stabilized Fluidized Beds, 22 SeparationScience and Technology, pp. 2111-2125 (1987). Although utilizing amagnetizable affinity particle, the particle used is of the non-porous,pellicular type. In such a system, only surface adsorption is possible.

Although affinity interactions can yield superb selectivity, it is anexpensive technique for separating proteins. The great selectivity seenmeans that only single-protein specific particles can be prepared.

As mentioned above, the use of magnetizable particles is not unknown inbiotechnology. The separation of proteins from mixtures by adsorptionunto magnetizable particles--either hydrophobic, affinity orion-exchange types--is often performed in batch preparations. Theparticle beads can be held in the bottom of a container with a magnetwhile excess solutions and wastes can be decanted out of the container.At the same time, the adsorbed proteins will remain with the particles.However, as is generally the case, for scale-up purposes it is usuallymore efficient to perform separations of this type on a column in a moreor less continuous process.

An example of a procedure used to prepare a Sepharose based magnetizableparticle using a ferrofluid is described in M. Mosbach and L. Andersson,Magnetic Ferrofluids for Preparation of Magnetic Polymers and TheirApplication in Affinity Chromatography, 270 Nature, pp. 259-261 (1977).

Combining the batch type adsorption processes into a column wouldprovide certain advantages. However, by taking advantage of the MSFB,protein isolation performance can be greatly enhanced. Unlike many otherseparation schemes, fluidized beds are quite conducive to scaling up. Noexamples of a magnetizable, porous and stationary particle bed forion-exchange adsorption/desorption or chromatography in an MSFB has beendisclosed in the literature.

SUMMARY OF THE INVENTION

This invention relates to a method and apparatus for performingliquid/solid separations utilizing magnetically stabilized fluidizedbeds ("MSFB"). More particularly, this invention describes an improvedmethod and system for separating proteins from a lysed cell mixtureutilizing magnetizable, porous, ion-exchange particles. Utilizing MSFBallows the use of increased flow rates, the easy manipulation of the"fluid" particle bed, and the elimination of bed fouling due to cellsolids.

A magnetizable particle is prepared according to the invention byplacing agarose based ion-exchange particles in a ferrofluid solutionfor a significant period of time. The particles isolated after thistreatment are suitable for use in this invention but have lost much oftheir ion-exchange capacity. Rather, these particles are discarded andthe original ferrofluid solution is contacted for a shorter period oftime with a fresh batch of agarose based ion-exchange particles.

The particles obtained from this second treatment have adsorbed asignificant amount of the magnetite suspended in the ferrofluid yet haveretained their ion-exchange capabilities. These particles are placed ina column that permits the flow of solution upwardly through the particlebed. The column is equipped with means for creating a magnetic fieldthat runs parallel to the flow of solution through the column and isessentially radially uniform in the area of the particle bed.

Isolating proteins manufactured and held within cells generally involvesa two step process after the cells have been lysed. The first step is aclarification of the lysate, where the cellular debris is removed fromthe mixture. The clarification is typically done by centrifugation ofthe lysate. The proteins in the clarified lysate are then isolated fromthe solution, often in a packed bed ion-exchange column or a batch-typeinteraction. Clarification always results in the reduction of proteinyield.

In the present invention, the lysate may be introduced into the bottomof the MSFB column without clarification. The rate of flow and thestrength of the magnetic field are adjusted so that a fluidized bed isgenerated. Due to the increased void area in the particle bed, the cellparticulate matter will generally pass through the bed without foulingthe particle bed or plugging flow. By increasing the magnetic fieldstrength, the flow rate at which the stable and random motion regimesmay be maintained is also increased. Increased flow rate can expeditethe adsorption process without a parallel increase in pressure buildup.

Following the introduction of a "charge" of cell products, the proteinin the mixture will be adsorbed into the porous ion-exchange particles.The amount of material to be included in the charge can be easilydetermined based on the flow rate and the quantity and binding capacityof the ion-exchange particles utilized. Once the cell matter has passedthrough the column, the ionic strength or pH of the solution flowingupwardly through the column is changed in order to alter the adsorptioncharacteristics of the system and allow the proteins to desorb from theparticle bed.

If an eluent gradient is utilized it is possible to obtain some proteinspecificity based on the ion-exchange characteristics of the individualproteins. The discharge from the top of the column may be collected andthe solutions will be protein enriched.

When introducing the lysate into the column, it is not essential thatthe particle bed be in the stable regime. Depending on various factors,such as the binding capacity and rate of the particles and the solutionflow rate, it may be desireable to maintain the bed in the random motionregime during the adsorption process. However, it is important tomaintain the particle bed in the stable regime when desorbing theprotein off the bed in order to assure that the protein is eluted fromthe column in a narrow band of solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of Stages A and B of anembodiment of the present invention.

FIG. 2 is a schematic diagram of an embodiment of the present inventionrepresenting the relationship of the magnetic field to the particlecontaining columns.

FIG. 3 is an elevational view of an apparatus used in an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention describes an improved means for separating proteins froma lysed cell mixture utilizing porous, magnetizable, ion-exchangeparticles. The particles are maintained in a magnetically stabilizedfluidized bed (MSFB) by the introduction of a radially uniform magneticfield with field lines parallel to the flow of solution. More generally,this invention describes a method and system for separating certainchemical species from a mixture containing other chemicals and suspendedparticles by the use of adsorption/desorption or chromatographicprocesses combined with stationary MSFB's.

A brief description of FIG. 1 will be used as an introduction to thevarious aspects of the present invention. FIG. 1 shows the two basicstages of the invention that utilize a MSFB. The support column 10 is acylindrical tube held in a vertical position. In order to obtain astable fluidized bed condition, it is imperative that the column be heldin a vertical position and that the solution flow is also in a verticaldirection.

The column 10 contains the magnetizable, ion-exchange particles 12. Theparticles 12 will be described in more detail below. The entire volumeof the column's interior space is not filled by the particle bed. One ofthe characteristics of MSFB is that the void volume of the particle bedis increased. It is, therefore, necessary to have a significant amountof column volume when charging the particle bed.

In one embodiment, this invention relates to a method of isolating aspecific protein or proteins from an impure feed solution. The feedsolution will typically contain microscopic particles, such as whole andlysed cell components, as well as other cell constituents.

Before introducing the protein feed solution onto the column, the stableor random motion MSFB regimes must be first obtained. These MSFB regimescan be obtained by either of two routes. The magnetic field may beapplied and the solution flow increased above the point of incipientfluidization, or the velocity can be raised until a random motion orbubbling fluidized bed is achieved and then the magnetic field isapplied. Determining what is the optimal magnetic field strength andsolution flow rate velocity is discussed below. In some cases, therandom motion regime may be preferred to the stable regime.

Once the designed MSFB regime has been attained, the protein feedsolution will generally be applied to the column as seen in Stage A ofFIG. 1. Due to the increased viscosity of the lysate, the flow rate ofthe solution must be decreased as the lysate is introduced onto thecolumn. This decrease in velocity must be anticipated in order to assurethat the particle bed be in the desired regime during the adsorptionstage of the process. The feed solution is generally introduced in apulse in a low ionic strength solvent. The proteins in the feed solutionwill adsorb to the ion-exchange sites of the particle beads. The cellwaste and other products in the cell lysate will pass through the columnin the solvent stream and may either be further treated or discarded.Because of the significantly increased void volume of the particle bed,the particulate matter in the cell will not tend to clog up the system.

The output of the column may be monitored by any number of types ofdetectors 14. A commonly used detector would be a UV-VIS spectrometricdetector whereby the outlet stream is continuously monitored at aparticular preset wavelength. During the first stage of the process, thedetector can be set to monitor if there has been any protein"breakthrough," or simply to monitor when the cell wastes have beeneluted from the column.

In Stage B of the process, the ionic strength or pH gradient of thesolution, which does not include any lysate, flowing through the columnis altered. This change should be accomplished while the column remainsin a MSFB regime. Regardless of the regime the bed was in during Stage Aof the process, the particle bed must be in the stable MSFB regimeduring Stage B. As the ionic strength of the solution increases or thepH changes, the ionic forces that bind the proteins to the ion-exchangesites of the particles are no longer as strong as the forces that drivethe proteins into solution. By utilizing an ionic strength or pHgradient, individual proteins will desorb off the resin at differentionic strengths. By this means, some protein separation can also beobtained based on the ionic characteristics of the individual proteins.Again, the detector monitoring the column output may be used todetermine what portions of solution exiting the column contain theisolated protein(s).

In some situations, it may be desirable to operate the column in astandard packed bed manner to perform the Stage B operation. If solutionflow is introduced from the top of the column and exits the bottom, theelution of proteins will occur in the classic manner.

FIG. 2 shows the orientation of the magnetic field 16 in the region ofthe particle bed 18 within the column 10. As can be seen, the magneticfield 16 runs axially in the region of the column, parallel to the flowof solution through the column. The appropriate magnetic field may begenerated by utilizing horizontally held magnet coils that surround thecolumn. In this manner, the magnetic field will be generally uniformacross any radial cross-section of the particle bed 18. It is this typeof magnetic field that allows for the optimal establishment of an MSFB.It would also be possible to use permanent magnets to generate thedesired magnetic field.

FIG. 3 shows an embodiment of the apparatus used in the presentinvention. The apparatus 20, consists of a glass cylinder 22 that may bevaried in capacity by the placement of stopper 24. The bottom of thecylinder 22 is defined by a frit 26 that supports the ion-exchangeparticles 12.

Exiting through the stopper 24, and in fluid communication with theinterior of the cylinder is liquid outlet line 28. Note that a voidexists between the top of the particle bed 12 and the bottom of thestopper 24.

Below the frit 26, the column chamber is in fluid communication with aglass funnel 30 that is held against the frit 26. An o-ring 32 betweenthe funnel 30 and the frit 26 assures that an environmental seal isformed.

The glass funnel 30, has two inlet or solution introduction orifices.The first inlet 34 is in fluid communication with the source of solventsolution. A pump (not shown) may be employed to deliver solution to thefirst inlet 34. The second inlet 36 is shown covered by a rubber septum38. The protein feed solution may be introduced into the solvent, andinto the cylinder 22, from this inlet 34 via a syringe or other means.

The entire apparatus is surrounded by two magnet coils. One set of coils40 is held horizontally at a position somewhat above the particle bed 12and the second set of coils 42 is held horizontally at a positionsomewhat below such bed 12.

The embodiment of the apparatus of the invention utilized to generatethe experimental results described below has the following dimensions.The diameter of the cylinder 22 is 1 inch, with a full height of 5inches, the placement of the stopper 24 being adjustable within thecylinder 22, and the volume of the cylinder being about 75 ml. Theexterior diameter of the magnet coils 40,42 is about 6 inches and thecoils are held about 3 inches from each other. The magnet coils 40,42each consist of 325 turns of 18 gauge copper wire. The frit 26 utilizedis 0.25 inches thick with about 45 micrometer pores.

The preferred magnetizable, porous ion-exchange particles used in thepresent invention are prepared through a multistep process. One startingmaterial is an agarose-based ion-exchange resin. The preferred materialutilized is S-Sepharose Fast Flow manufactured by Pharmacia ofPiscataway, N.J. The other solution used is a commercially availableferrofluid. A ferrofluid is an aqueous solution of small suspendedmagnetite particles (Fe₃ O₄) that are coated with a cationic dispersingagent that prevents the magnetite particles from aggregating insolution. A ferrofluid may be purchased from Ferrofluids of Nashua,Mass.

The intimate mixture of the ion-exchange particles and theferrofluid--following different procedures--may yield two distinctmagnetizable products. An agitated solution of the ion-exchangeparticles in the ferrofluid for an extended period of time followed bydecantion of the excess ferrofluid yields a first type of magnetizableparticle (Type I). On visual inspection, these particles have a darkbrown but transparent appearance (the untreated S-Sepharose Fast Flow isclear and transparent). The original ferrofluid should contain at leastabout 1.5% magnetite.

However, if the ferrofluid decanted from the mixture is subsequentlymixed with a fresh batch of the ion-exchange particles, the resultanttreated particles have a different appearance. This particle type (TypeII) is brown and opaque, and is not smooth on its exterior surface. Ahigh magnetic susceptibility, greater than 1×10⁻⁴ cgs units, was seenfor both types of particles. Magnetic susceptibility was measured as apacked bed, water filling the void volume of the bed.

Examination of the S-Sepharose starting material and the Type I and TypeII magnetizable particles by scanning electron microscope shows adramatic difference between the Type I and Type II particles. Theuntreated S-Sepharose and the Type I particles appear nearly identicalat magnifications of about one thousand times. The particles have asmooth spherical surface. At the same magnification, the Type IIparticle has a drastically different appearance. The surface of theparticles is rough and cratered, and the rough coating appears tocompletely cover the surface of the S-Sepharose particle.

The Type I particles have a decreased protein binding capacity relativeto the untreated ion-exchange particles. The Type II particles, however,have an essentially identical protein adsorption capacity as theuntreated S-Sepharose starting material. The preferred magnetized,porous, ion-exchange particles, having a high magnetic susceptibilityand excellent protein binding characteristics, are the Type IIparticles.

Based on the results of the electron scanning microscope and proteinbinding experiments, it seems clear that in the Type I particles, themagnetite has entered the pore space of the particles and is blockingaccess to the ion-exchange sites therein. In the Type II particles, themagnetite is forming a surface covering, but the surface layer is notpreventing the access of proteins into the pores of the particles.

A MSFB prepared utilizing the Type II magnetized, porous, ion-exchangeresin may be used to isolate proteins from a solution byadsorption/desorption as described above in conjunction with FIG. 1. Ofcourse, the present invention need not be so restricted as to the type aseparation that can be performed or the particles utilized. For example,the isolation of certain organic chemical species from a reactionmixture could be susceptible to the use of MSFB utilizing magnetizable,porous stationary phases.

Possible explanations for the formation of the two different types oftreated particles are only speculative. One mechanism, not intended tolimit the scope of this invention, is that during the initial step, thesmaller magnetite particles enter the pores of the S-Sepharose particlesand associate with the ion-exchange sites within the material. In thesecond treatment, the remaining larger magnetite particles adhere ontothe surface of the particles, yet aggregate in such a manner that accessto the interior of the particles is not blocked.

As shown in the following examples, the Type II particles of thisinvention contain at least 5% by weight excess of 1×10⁻⁴ cgs units. Inaddition, the particles have a binding capacity that has been decreasedfrom that of the untreated particle by less than 10%.

It is envisioned that the process utilized to create the Type IIion-exchange particles would lead to the same desireable characteristicsif applied to any other generally available stationary phases used inchromatography or adsorption/desorption processes. These desireableproperties and the method for obtaining such particles is not limited toion-exchange materials as described herein.

A key element of the present invention is the use of porous rather thanpellicular adsorption particles. Porous particles have a greatlyincreased capacity for adsorbing molecules. The Type II particlesidentified and described above have excellent magnetizabilitycharacteristics, yet also are porous and have a significant number ofinterparticle adsorption sites.

An interesting, yet currently unexplained, phenomena, is the fluidizedbed properties of the Type II particles in the absence of a magneticfield. It is significantly easier to maintain the treated particles in arandom motion fluidized bed regime than the untreated S-Sepharoseparticles.

The examples given below relate specific experiments that have beenperformed relating to the production and characteristics of themagnetizable, porous, ion-exchange particles and the performance of theparticles in MSFB adsorption/desorption isolation of a proteincontaining solution.

EXAMPLE 1 Preparation of Type I Resin

6.8 ml of moist S-Sepharoase Fast Flow (90-165 micrometer particles) wasadded to 7 ml of ferrofluid (#EMG 607, about 1.7% by volume magnetite).This mixture was stirred for 19 hours. The particles isolated from thismixture are dark brown and translucent. These particles were found tohave a magnetic susceptibility of 3×10⁻⁴ cgs units. Based on the dryweight of the particles, the material contained 15.1% magnetite (Fe₃O₄).

EXAMPLE 2 Preparation of Type II Resin

The ferrofluid solution from the mixture of Example 1 is mixed with afresh batch of 4.5 ml of the moist S-Sephrose particles and stirred for4 hours. The particles isolated from this mixture are dark brown andopaque, and the surface appears "roughened" or as if coated with a finebrown powder. These particles were found to have a magneticsusceptibility of 4.5×10⁻⁴ cgs units. Based on the dry weight of theparticles, the material contained 9.3% magnetite.

EXAMPLE 3 Protein Binding Capacity

The protein binding capacities of the particles prepared in Example 1and Example 2, as well as a sample of untreated S-Sepharose, weredetermined by exposing 1 ml of the various moistened particles to aCytochrome-C containing solution. 20 ml of the solution contained 1 mgof the protein to each ml of solution, and the solution was held at pH7. The change in concentration was monitored by visible absorptionspectrometry. The results shown below indicate the milligrams ofCytochrome-C adsorbed by the particles after 5 minutes, a total of 20 mgavailable.

    ______________________________________                                        Particle           mg. bound                                                  ______________________________________                                        Untreated S-Sepharose                                                                            19.7                                                       Type I treated      0.9                                                       Type II treated    19.4                                                       ______________________________________                                    

EXAMPLE 4 Establishment of MSFB utilizing Type II Particles

The apparatus shown in FIG. 3, and described above, was used to create amagnetically stabilized fluidized bed. A study was made whereby themagnetic field and the solution flow rate were varied. For a givenmagnetic field strength, the flow rate was increased until thetransition between the stable and random motion regimes was detected.The results of these experiment are shown below:

    ______________________________________                                        Transition Velocity                                                                            Magnetic Field                                               Linear Flow (cm/min)                                                                           (gauss)                                                      ______________________________________                                        .58              41                                                           .73              56                                                           .83              70                                                           .96              88                                                           1.08             105                                                          ______________________________________                                    

EXAMPLE 5 Adsorption/Desorption of Cytochrome-C

An experiment wherein the Type II resin as prepared in Example 2 is heldin a MSFB regime and a single protein (Cytochrome-C) is first adsorbedand then desorbed from the particles was performed. The apparatus shownin FIG. 3 and described above was utilized.

The solvent utilized in Stage A of the process consisted of an aqueous0.025M phosphate buffer, pH 7. The desorption solvent in Stage B was apH 7, 0.5M NaCl solution. The flow rate employed was 0.28 cm/min, themagnetic field was 77 gauss, and the bed height of the particle was 0.9cm. The results of three runs is shown below:

    ______________________________________                                                      Run 1   Run 2   Run 3                                           ______________________________________                                        protein feed (mg)                                                                             6.56      9.34    7.12                                        protein loaded (mg)                                                                           6.11      6.26    4.88                                        protein recovered (mg)                                                                        5.28      5.28    4.55                                        ______________________________________                                    

As indicated in the preceding Examples, an improved means for isolatingproteins from a mixture utilizing stationary MSFB and a magnetizable,porous, ion-exchange particle bed is described. Of course, furtherapplications of the invention beyond the Examples given are includedwith the scope of this invention. The claims below should not be limitedby the embodiments described above.

We claim:
 1. A magnetically stabilized fluidized bed system forisolating certain chemical species from a solution of chemicals andsolids comprising:a vertically held column having a solution inlet inits bottom portion and a solution outlet in its top portion;magnetizable, porous particles capable of adsorbing said certainchemical species, within said column, said particles including magnetiteadhering predominantly to the outer surface of said particles withoutsignificantly affecting the adsorption capabilities of said particles;magnet means for generating a radially-uniform magnetic field in theregion of said column containing said particles; means for introducingliquid into said inlet and means for receiving liquid from said outlet,whereby the flow of liquid from said inlet, through said column and outsaid outlet, creates a stationary bed of said particles.
 2. The systemof claim 1 wherein said particles contain at least about 5% by weightmagnetite.
 3. The system of claim 2 wherein said magnetite adheres tothe outer surface of said particles without significantly affecting theadsorptive capabilities of said particles.
 4. The system of claim 1wherein said particles have a magnetic susceptibility of at least about1.0×10⁻⁴ cgs units.
 5. The system of claim 1 wherein said particles haveion-exchange properties.
 6. The system of claim 1 wherein said magnetmeans consist of at least one horizontally held magnet coil surroundingsaid column.
 7. A magnetically stabilized fluidized bed system forisolating certain chemical species from a solution of chemicals andsolids comprising:a vertically held column having a solution inlet inits bottom portion and a solution outlet in its top portion;magnetizable, porous particles capable of absorbing said certainchemical species, within said column, said particles containing at leastabout 5% by weight magnetite, having a magnetic susceptibility of atleast about 1.0×10⁻⁴ cgs units, said magnetite generally constitutingthe outer surface of said particles without significantly affecting theadsorption capabilities of said particles; magnet means for generating aradially-uniform magnetic field in the region of said column containingsaid particles; and means for introducing liquid into said inlet andmeans for receiving liquid from said outlet, whereby the flow of liquidfrom said inlet, through said column and out said outlet, creates astationary bed of said particles.
 8. A magnetizable, porous particle foruse with magnetically stabilized fluidized bed separations comprising aporous particle without significantly affecting the adsorptioncapabilities of said particles, at least about 5% by weight magnetite,said magnetite adhering predominantly to the outer surface of saidparticle capable of adsorbing certain chemical species, having amagnetic susceptibility of at least about 1×10⁻⁴ cgs units, and havingsubstantially unhindered access into the pores of said particle.
 9. Theparticle of claim 8 wherein adsorption sites are contained substantiallywithin said pores of said particles.
 10. The particle of claim 9 whereinsaid adsorption sites are ion-exchanged sites.
 11. A particle capable ofadsorbing certain chemical species for use with magnetically stabilizedfluidized bed separation comprising:a porous particle whereinsubstantially all its adsorption sites are within the pores of saidparticle; and a continuous and porous layer of magnetite on the surfaceof said particle, wherein access into said pores is not significantlyrestricted.
 12. The particle of claim 11 wherein the magnetite coatingcomprises at least about 5% by weight of said particle.
 13. The particleof claim 11 having a magnetic susceptibility of at least about 1.0×10⁻⁴cgs units.
 14. The particle of claim 11 having a chemical bindingcapacity within 10% of the binding capacity of the uncoated particle.